The space station's crew was rudely awakened Saturday night when an electrical spike caused a pump that drives coolant through most of the vehicle's electrical systems to fail, setting off warnings inside the station and a round-the-clock effort on the ground at NASA to figure out what to do.

Although the pump's failure wasn't a surprise in the sense that NASA had planned for it, according to NASA program manager Mike Suffredini, it happened earlier than NASA expected. (The pump was expected to last for a mean time of about 100,000 hours, and it failed after about 80,000 hours).

The failure also means the crew has limited backup if the second pump fails. NASA is working out what would happen if the second pump fails as we speak.

A space walk was already planned for Thursday, so two crew members will now be using that walk to try to replace the failed pump, which flight director Courtenay McMillan describes as "a big unwieldy object" that may measure three feet by six feet.

Doing a space walk on short notice is risky too. Today, two astronauts have been submerged in NASA's Neutral Buoyancy Lab -- a large water tank near Johnson Space Center in Houston where movement in space can be replicated -- so they can rehearse the space walk and work out step-by-step procedures for the crew. There will be a second training run in the tank tomorrow, and possibly a third run before the second space walk on Sunday.

On the space station, meanwhile, the crew has powered down all unnecessary systems, including lights and heaters.

NASA maintains the crew is not and never was in any danger -- just short of sleep. But "if we lose the next cooling system we can't cool most of the components on board the ISS, which would be a significant challenge to the team," Suffredini said at a press conference today. "The team would just as soon not be in that position, so there's all haste to get this one done and put ourselves back in a redundancy position."

He said NASA is considering designing the next pump with redundant motors.

NASA hasn't had a disaster since the Space Shuttle Columbia disintegrated over Texas while re-entering the atmosphere on February 1, 2003. Remember that a piece of foam insulation broke off the shuttle's external fuel tank and hit the shuttle's wing during launch. On re-entry, all seven crew members were killed.

Although this broken pump seems far less damaging than the broken foam, it is still dangerous. One question is how much the crew will be able to use the space station's robotic arm on the upcoming space walk to help them with their tasks. Another issue is how quickly the crew could decontaminate themselves if they're exposed to the ammonia coolant in the pump.

You can follow what's going on at NASA TV, which is broadcasting NASA's press conferences and the space walk, which is scheduled for Thursday at 5 a.m. Central Time.

UPDATE: The first space walk has now been postponed until Friday at 5:55 AM Central Time. The second space walk is scheduled for Monday.